Internet DRAFT - draft-iab-privacy-partitioning
draft-iab-privacy-partitioning
Network Working Group M. Kühlewind
Internet-Draft Ericsson Research
Intended status: Informational T. Pauly
Expires: 14 September 2023 Apple
C. A. Wood
Cloudflare
13 March 2023
Partitioning as an Architecture for Privacy
draft-iab-privacy-partitioning-01
Abstract
This document describes the principle of privacy partitioning, which
selectively spreads data and communication across multiple parties as
a means to improve the privacy by separating user identity from user
data. This document describes emerging patterns in protocols to
partition what data and metadata is revealed through protocol
interactions, provides common terminology, and discusses how to
analyze such models.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Internet Architecture
Board Internet Engineering Task Force mailing list (iab@iab.org),
which is archived at .
Source for this draft and an issue tracker can be found at
https://github.com/intarchboard/draft-obliviousness.
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This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 14 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Privacy Partitioning . . . . . . . . . . . . . . . . . . . . 4
2.1. Privacy Contexts . . . . . . . . . . . . . . . . . . . . 4
2.2. Context Separation . . . . . . . . . . . . . . . . . . . 7
2.3. Approaches to Partitioning . . . . . . . . . . . . . . . 7
3. A Survey of Protocols using Partitioning . . . . . . . . . . 8
3.1. CONNECT Proxying and MASQUE . . . . . . . . . . . . . . . 8
3.2. Oblivious HTTP and DNS . . . . . . . . . . . . . . . . . 12
3.3. Privacy Pass . . . . . . . . . . . . . . . . . . . . . . 13
3.4. Privacy Preserving Measurement . . . . . . . . . . . . . 14
4. Applying Privacy Partioning . . . . . . . . . . . . . . . . . 14
4.1. User-Identifying Information . . . . . . . . . . . . . . 15
4.2. Incorrect or Incomplete Partitioning . . . . . . . . . . 15
4.3. Identifying Information for Partitioning . . . . . . . . 16
5. Limits of Privacy Partitioning . . . . . . . . . . . . . . . 16
5.1. Violations by Collusion . . . . . . . . . . . . . . . . . 17
5.2. Violations by Insufficient Partitioning . . . . . . . . . 17
6. Partitioning Impacts . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Informative References . . . . . . . . . . . . . . . . . . . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Protocols such as TLS and IPsec provide a secure (authenticated and
encrypted) channel between two endpoints over which endpoints
transfer information. Encryption and authentication of data in
transit is necessary to protect information from being seen or
modified by parties other than the intended protocol participants.
As such, this kind of security is necessary for ensuring that
information transferred over these channels remain private.
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However, a secure channel between two endpoints is insufficient for
privacy of the endpoints themselves. In recent years, privacy
requirements have expanded beyond the need to protect data in transit
between two endpoints. Some examples of this expansion include:
* A user accessing a service on a website might not consent to
reveal their location, but if that service is able to observe the
client's IP address, it can learn something about the user's
location. This is problematic for privacy since the service can
link user data to the user's location.
* A user might want to be able to access content for which they are
authorized, such as a news article, without needing to have which
specific articles they read on their account being recorded. This
is problematic for privacy since the service can link user
activity to the user's account.
* A client device that needs to upload metrics to an aggregation
service might want to be able to contribute data to the system
without having their specific contributions being attribued to
them. This is problematic for privacy since the service can link
client contributions to the specific client.
The commonality in these examples is that clients want to interact
with or use a service without exposing too much user-specific or
identifying information to that service. In particular, separating
the user-specific identity information from user-specific data is
necessary for privacy. Thus, order to protect user privacy, it is
important to keep identity (who) and data (what) separate.
This document defines "privacy partitioning," sometimes also referred
to as the "decoupling principle" [DECOUPLING], as the general
technique used to separate the data and metadata visible to various
parties in network communication, with the aim of improving user
privacy. Partitioning is a spectrum and not a panacea. It is
difficult to guarantee there is no link between user-specific
identity and user-specific data. However, applied properly, privacy
partitioning helps ensure that user privacy violations becomes more
technically difficult to achieve over time.
Several IETF working groups are working on protocols or systems that
adhere to the principle of privacy partitioning, including OHAI,
MASQUE, Privacy Pass, and PPM. This document summarizes work in
those groups and describes a framework for reasoning about the
resulting privacy posture of different endpoints in practice.
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[RFC6973] discusses data minimization, especially in the context of
user identity and identity management systems. In these systems
usually an identify provider issues credentials that can be used to
access a service without revealing the user's identity by relying on
the authentication assertion from the identity provider (see
Section 6.1.4 of [RFC6973]). This describes a specific form of
privacy partitioning, similar as used for privacy pass (see
Section Section 3.3). Privacy partitioning as defined in this
document goes further, to consider different deployment models that
can create multiple contexts where data is minimized in each context.
2. Privacy Partitioning
For the purposes of user privacy, this document focuses on user-
specific information. This might include any identifying information
that is specific to a user, such as their email address or IP
address, or data about the user, such as their date of birth.
Informally, the goal of privacy partitioning is to ensure that each
party in a system beyond the user themselves only has access to one
type of user-specific information.
This is a simple application of the principle of least privilege,
wherein every party in a system only has access to the minimum amount
of information needed to fulfill their function. Privacy
partitioning advocates for this minimization by ensuring that
protocols, applications, and systems only reveal user-specific
information to parties that need access to the information for their
intended purpose.
Put simply, privacy partitioning aims to separate _who_ someone is
from _what_ they do. In the rest of this section, we describe how
privacy partitioning can be used to achieve this goal.
2.1. Privacy Contexts
Each piece of user-specific information exists within some context,
where a context is abstractly defined as a set of data and metadata
and the entities that share access to that information. In order to
prevent correlation of user-specific information across contexts,
partitions need to ensure that any single entity (other than the
client itself) does not participate in more than one context where
the information is visible.
[RFC6973] discusses the importance of identifiers in reducing
correlation as a way of improving privacy:
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| Correlation is the combination of various pieces of information
| related to an individual or that obtain that characteristic when
| combined... Correlation is closely related to identification.
| Internet protocols can facilitate correlation by allowing
| individuals' activities to be tracked and combined over time.
|
| Pseudonymity is strengthened when less personal data can be linked
| to the pseudonym; when the same pseudonym is used less often and
| across fewer contexts; and when independently chosen pseudonyms
| are more frequently used for new actions (making them, from an
| observer's or attacker's perspective, unlinkable).
Context separation is foundational to privacy partitioning and
reducing correlation. As an example, consider an unencrypted HTTP
session over TCP, wherein the context includes both the content of
the transaction as well as any metadata from the transport and IP
headers; and the participants include the client, routers, other
network middleboxes, intermediaries, and server.
+-------------------------------------------------------------------+
| Context A |
| +--------+ +-----------+ +--------+ |
| | +------HTTP------+ +--------------+ | |
| | Client | | Middlebox | | Server | |
| | +------TCP-------+ +--------------+ | |
| +--------+ flow +-----------+ +--------+ |
| |
+-------------------------------------------------------------------+
Figure 1: Diagram of a basic unencrypted client-to-server
connection with middleboxes
Adding TLS encryption to the HTTP session is a simple partitioning
technique that splits the previous context into two separate
contexts: the content of the transaction is now only visible to the
client, TLS-terminating intermediaries, and server; while the
metadata in transport and IP headers remain in the original context.
In this scenario, without any further partitioning, the entities that
participate in both contexts can allow the data in both contexts to
be correlated.
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+-------------------------------------------------------------------+
| Context A |
| +--------+ +--------+ |
| | | | | |
| | Client +-------------------HTTPS-------------------+ Server | |
| | | | | |
| +--------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Context B |
| +--------+ +-----------+ +--------+ |
| | | | | | | |
| | Client +-------TCP------+ Middlebox +--------------+ Server | |
| | | flow | | | | |
| +--------+ +-----------+ +--------+ |
| |
+-------------------------------------------------------------------+
Figure 2: Diagram of how adding encryption splits the context
into two
Another way to create a partition is to simply use separate
connections. For example, to split two separate HTTP requests from
one another, a client could issue the requests on separate TCP
connections, each on a different network, and at different times; and
avoid including obvious identifiers like HTTP cookies across the
requests.
+-------------------------------------------------------------------+
| Context A |
| +--------+ +-----------+ +--------+ |
| | | IP A | | | | |
| | Client +-------TCP------+ Middlebox +--------------+ Server | |
| | | flow A | A | | | |
| +--------+ +-----------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Context B |
| +--------+ +-----------+ +--------+ |
| | | IP B | | | | |
| | Client +-------TCP------+ Middlebox +--------------+ Server | |
| | | flow B | B | | | |
| +--------+ +-----------+ +--------+ |
| |
+-------------------------------------------------------------------+
Figure 3: Diagram of making separate connections to generate
separate contexts
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Using separate connections to create separate contexts can reduce or
eliminate the ability of specific parties to correlate activity
across contexts. However, any identifier at any layer that is common
across different contexts can be used as a way to correlate activity.
Beyond IP addresses, many other factors can be used together to
create a fingerprint of a specific device (such as MAC addresses,
device properties, software properties and behavior, application
state, etc).
2.2. Context Separation
In order to define and analyze how various partitioning techniques
work, the boundaries of what is being partitioned need to be
established. This is the role of context separation. In particular,
in order to prevent correlation of user-specific information across
contexts, partitions need to ensure that any single entity (other
than the client itself) does not participate in contexts where both
identities are visible.
Context separation can be achieved in different ways, for example,
over time, across network paths, based on (en)coding, etc. The
privacy-oriented protocols described in this document generally
involve more complex partitioning, but the techniques to partition
communication contexts still employ the same techniques:
1. Encryption allows partitioning of contexts within a given network
path.
2. Using separate connections across time or space allow
partitioning of contexts for different application transactions.
These techniques are frequently used in conjunction for context
separation. For example, encrypting an HTTP exchange might prevent a
network middlebox that sees a client IP address from seeing the user
account identity, but it doesn't prevent the TLS-terminating server
from observing both identities and correlating them. As such,
preventing correlation requires separating contexts, such as by using
proxying to conceal a client IP address that would otherwise be used
as an identifier.
2.3. Approaches to Partitioning
While all of the partitioning protocols described in this document
create separate contexts using encryption and/or connection
separation, each one has a unique approach that results in different
sets of contexts. Since many of these protocols are new, it is yet
to be seen how each approach will be used at scale across the
Internet, and what new models will emerge in the future.
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There are multiple factors that lead to a diversity in approaches to
partitioning, including:
* Adding privacy partitioning to existing protocol ecosystems places
requirements and constraints on how contexts are constructed.
CONNECT-style proxying is intended to work with servers that are
unaware of privacy contexts, requiring more intermediaries to
provide strong separation guarantees. Oblivious HTTP, on the
other hand, assumes servers that cooperate with context
separation, and thus reduces the overall number of elements in the
solution.
* Whether or not information exchange needs to happen
bidirectionally in an interactive fashion determines how contexts
can be separated. Some use cases, like metrics collection for
PPM, can occur with information flowing only from clients to
servers, and can function even when clients are no longer
connected. Privacy Pass is an example of a case that can be
either interactive or not, depending on if tokens can be cached
and reused. CONNECT-style proxying and Oblivious HTTP often
require bidirectional and interactive communication.
* The degree to which contexts need to be partitioned depends in
part on the client's threat models and level of trust in various
protocol participants. For example, in Oblivious HTTP, clients
allow relays to learn that clients are accessing a particular
application-specific gateway. If clients do not trust relays with
this information, they can instead use a multi-hop CONNECT-style
proxy approach wherein no single party learns whether specific
clients are accessing a specific application. This is the default
trust model for systems like Tor, where multiple hops are used to
drive down the probability of privacy violations due to collusion
or related attacks.
3. A Survey of Protocols using Partitioning
The following section discusses currently on-going work in the IETF
that is applying privacy partitioning.
3.1. CONNECT Proxying and MASQUE
HTTP forward proxies, when using encryption on the connection between
the client and the proxy, provide privacy partitioning by separating
a connection into multiple segments. When connections to targets via
the proxy themselves are encrypted, the proxy cannot see the end-to-
end content. HTTP has historically supported forward proxying for
TCP-like streams via the CONNECT method. More recently, the
Multiplexed Application Substrate over QUIC Encryption (MASQUE)
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working group has developed protocols to similarly proxy UDP
[CONNECT-UDP] and IP packets [CONNECT-IP] based on tunneling.
In a single-proxy setup there is a tunnel connection between the
client and proxy and an end-to-end connection that is tunnelled
between the client and target. This setup, as shown in the figure
below, partitions communication into:
* a Client-to-Proxy context, which contains the transport metadata
between the client and the target, and the request to the proxy to
open a connection to the target;
* a Client-to-Target proxied context, which is the end-to-end data
to the target that is also visible to the proxy, such as a TLS
session;
* a Client-to-Target encrypted context, which contains the end-to-
end content with the TLS session to the target, such as HTTP
content;
* and a Proxy-to-Target context, which for TCP and UDP proxying
contains any packet header information that is added or modified
by the proxy, e.g., the IP and TCP/UDP headers.
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+-------------------------------------------------------------------+
| Client-to-Target Encrypted Context |
| +--------+ +--------+ |
| | | | | |
| | Client +------------------HTTPS--------------------+ Target | |
| | | content | | |
| +--------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Client-to-Target Proxied Context |
| +--------+ +-----------+ +--------+ |
| | | | | | | |
| | Client +----Proxied-----+ Proxy +--------------+ Target | |
| | | TLS flow | | | | |
| +--------+ +-----------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Client-to-Proxy Context |
| +--------+ +-----------+ |
| | | | | |
| | Client +---Transport----+ Proxy | |
| | | flow | | |
| +--------+ +-----------+ |
| |
+-------------------------------------------------------------------+
| Proxy-to-Target Context |
| +-----------+ +--------+ |
| | | | | |
| | Proxy +--Transport---+ Target | |
| | | flow | | |
| +-----------+ +--------+ |
| |
+-------------------------------------------------------------------+
Figure 4: Diagram of one-hop proxy contexts
Using two (or more) proxies provides better privacy partitioning. In
particular, with two proxies, each proxy sees the Client metadata,
but not the Target; the Target, but not the Client metadata; or
neither.
+-------------------------------------------------------------------+
| Client-to-Target Encrypted Context |
| +--------+ +--------+ |
| | | | | |
| | Client +------------------HTTPS--------------------+ Target | |
| | | content | | |
| +--------+ +--------+ |
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| |
+-------------------------------------------------------------------+
| Client-to-Target Proxied Context |
| +--------+ +-------+ +--------+ |
| | | | | | | |
| | Client +----------Proxied----------+ Proxy +-------+ Target | |
| | | TLS flow | B | | | |
| +--------+ +-------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Client-to-Proxy B Context |
| +--------+ +-------+ +-------+ |
| | | | | | | |
| | Client +---------+ Proxy +---------+ Proxy | |
| | | | A | | B | |
| +--------+ +-------+ +-------+ |
| |
+-------------------------------------------------------------------+
| Client-to-Proxy A Context |
| +--------+ +-------+ |
| | | | | |
| | Client +---------+ Proxy | |
| | | | A | |
| +--------+ +-------+ |
| |
+-------------------------------------------------------------------+
| Proxy A-to-Proxy B Context |
| +-------+ +-------+ |
| | | | | |
| | Proxy +---------+ Proxy | |
| | A | | B | |
| +-------+ +-------+ |
| |
+-------------------------------------------------------------------+
| Proxy B-to-Target Context |
| +-------+ +--------+ |
| | | | | |
| | Proxy +-------+ Target | |
| | B | | | |
| +-------+ +--------+ |
| |
+-------------------------------------------------------------------+
Figure 5: Diagram of two-hop proxy contexts
Forward proxying, such as the protocols developed in MASQUE, uses
both encryption (via TLS) and separation of connections (via proxy
hops that see only the next hop) to achieve privacy partitioning.
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3.2. Oblivious HTTP and DNS
Oblivious HTTP [OHTTP], developed in the Oblivious HTTP Application
Intermediation (OHAI) working group, adds per-message encryption to
HTTP exchanges through a relay system. Clients send requests through
an Oblivious Relay, which cannot read message contents, to an
Oblivious Gateway, which can decrypt the messages but cannot
communicate directly with the client or observe client metadata like
IP address. Oblivious HTTP relies on Hybrid Public Key Encryption
[HPKE] to perform encryption.
Oblivious HTTP uses both encryption and separation of connections to
achieve privacy partitioning. The end-to-end messages are encrypted
between the Client and Gateway (forming a Client-to-Gateway context),
and the connections are separated into a Client-to-Relay context and
a Relay-to-Gateway context. It is also important to note that the
Relay-to-Gateway connection can be a single connection, even if the
Relay has many separate Clients. This provides better anonymity by
making the pseudonym presented by the Relay to be shared across many
Clients.
+-------------------------------------------------------------------+
| Client-to-Target Context |
| +--------+ +---------+ +--------+ |
| | | | | | | |
| | Client +---------------------------+ Gateway +-----+ Target | |
| | | | | | | |
| +--------+ +---------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Client-to-Gateway Context |
| +--------+ +-------+ +---------+ |
| | | | | | | |
| | Client +---------+ Relay +---------+ Gateway | |
| | | | | | | |
| +--------+ +-------+ +---------+ |
| |
+-------------------------------------------------------------------+
| Client-to-Relay Context |
| +--------+ +-------+ |
| | | | | |
| | Client +---------+ Relay | |
| | | | | |
| +--------+ +-------+ |
| |
+-------------------------------------------------------------------+
Figure 6: Diagram of Oblivious HTTP contexts
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Oblivious DNS over HTTPS [ODOH] applies the same principle as
Oblivious HTTP, but operates on DNS messages only. As a precursor to
the more generalized Oblivious HTTP, it relies on the same HPKE
cryptographic primitives, and can be analyzed in the same way.
3.3. Privacy Pass
Privacy Pass is an architecture [PRIVACYPASS] and set of protocols
being developed in the Privacy Pass working group that allow clients
to present proof of verification in an anonymous and unlinkable
fashion, via tokens. These tokens originally were designed as a way
to prove that a client had solved a CAPTCHA, but can be applied to
other types of user or device attestation checks as well. In Privacy
Pass, clients interact with an attester and issuer for the purposes
of issuing a token, and clients then interact with an origin server
to redeeem said token.
In Privacy Pass, privacy partitioning is achieved with cryptographic
protection (in the form of blind signature protocols or similar) and
separation of connections across two contexts: a "redemption context"
between clients an origins (servers that request and receive tokens),
and an "issuance context" between clients, attestation servers, and
token issuance servers. The cryptographic protection ensures that
information revealed during the issuance context is separated from
information revealed during the redemption context.
+-------------------------------------------------------------------+
| Redemption Context |
| +--------+ +--------+ |
| | | | | |
| | Origin +---------+ Client | |
| | | | | |
| +--------+ +--------+ |
| |
+-------------------------------------------------------------------+
| Issuance Context |
| +--------+ +----------+ +--------+ |
| | | | | | | |
| | Client +------+ Attester +------+ Issuer | |
| | | | | | | |
| +--------+ +----------+ +--------+ |
| |
+-------------------------------------------------------------------+
Figure 7: Diagram of contexts in Privacy Pass
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3.4. Privacy Preserving Measurement
The Privacy Preserving Measurement (PPM) working group is chartered
to develop protocols and systems that help a data aggregation or
collection server (or multiple, non-colluding servers) compute
aggregate values without learning the value of any one client's
individual measurement. Distributed Aggregation Protocol (DAP) is
the primary working item of the group.
At a high level, DAP uses a combination of cryptographic protection
(in the form of secret sharing amongst non-colluding servers) to
establish two contexts: an "upload context" between clients and non-
colluding aggregation servers wherein aggregation servers possibly
learn client identity but nothing about their individual measurement
reports, and a "collect context" wherein a collector learns aggregate
measurement results and nothing about individual client data.
+-------------------------------------+--------------------+
| Upload Context | Collect Context |
| +------------+ | |
| +----->| Helper | | |
| +--------+ | +------------+ | |
| | +---+ ^ | +-----------+ |
| | Client | | | | Collector | |
| | +---+ v | +-----+-----+ |
| +--------+ | +------------+ | | |
| +----->| Leader |<-----------+ |
| +------------+ | |
+-------------------------------------+--------------------+
Figure 8: Diagram of contexts in DAP
4. Applying Privacy Partioning
Applying privacy partitioning to an existing or new system or
protocol requires the following steps:
1. Identify the types of information used or exposed in a system or
protocol, some of which can be used to identify a user or
correlate to other contexts.
2. Partition data to minimize the amount of user-identifying or
correlatable information in any given context to only include
what is necessary for that context, and prevent sharing of data
across contexts wherever possible.
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The most impactful types of information to partition are (a) user-
identifying information, such as user identity or identities
(including account names or IP addresses) that can be linked and (b)
non-user-identifying information (including content a user generates
or accesses), which can be often sensitive when combined with user
identity.
In this section, we discuss considerations for partitioning these
types of information.
4.1. User-Identifying Information
User data can itself be user-identifying, in which case it should be
treated as an identifier. For example, Oblivious DoH and Oblivious
HTTP partition the client IP address and client request data into
separate contexts, thereby ensuring that no entity beyond the client
can observe both. Collusion across contexts could reverse this
partitioning, but can also promote non-user-identifying information
to user-identifying. For example, in CONNECT proxy systems that use
QUIC, the QUIC connection ID is inherently non-user-identifying since
it is generated randomly ([QUIC], Section 5.1). However, if combined
with another context that has user-identifying information such as
the client IP address, the QUIC connection ID can become user-
identifying information.
Some information is innate to client user-agents, including details
of implementation of protocols in hardware and software, and network
location. This information can be used to construct user-identifying
information, which is a process sometimes referred to as
fingerprinting. Depending on the application and system constraints,
users may not be able to prevent fingerprinting in privacy contexts.
As a result, fingerprinting information, when combined with non-user-
identifying user data, could promote user data to user-identifying
information.
4.2. Incorrect or Incomplete Partitioning
Privacy partitioning can be applied incorrectly or incompletely.
Contexts may contain more user-identifying information than desired,
or some information in a context may be more user-identifying than
intended. Moreover, splitting user-identifying information over
multiple contexts has to be done with care, as creating more contexts
can increase the number of entities that need to be trusted to not
collude. Nevertheless, partitions can help improve the client's
privacy posture when applied carefully.
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Evaluating and qualifying the resulting privacy of a system or
protocol that applies privacy partitioning depends on the contexts
that exist and types of user-identifying information in each context.
Such evaluation is helpful for identifying ways in which systems or
protocols can improve their privacy posture. For example, consider
DNS-over-HTTPS [DOH], which produces a single context which contains
both the client IP address and client query. One application of
privacy partitioning results in ODoH, which produces two contexts,
one with the client IP address and the other with the client query.
4.3. Identifying Information for Partitioning
Recognizing potential appliations of privacy partitoning requires
identifying the contexts in use, the information exposed in a
context, and the intent of information exposed in a context.
Unfortunately, determing what information to include in a given
context is a nontrivial task. In principle, the information
contained in a context should be fit for purpose. As such, new
systems or protocols developed should aim to ensure that all
information exposed in a context serves as few purposes as possible.
Designing with this principle from the start helps mitigate issues
that arise if users of the system or protocol inadvertently ossify on
the information available in contexts. Legacy systems that have
ossified on information available in contexts may be difficult to
change in practice. As an example, many existing anti-abuse systems
depend on some notion of client identity such as client IP address,
coupled with client data, to provide value. Partitioning contexts in
these systems such that they no longer see the client identity
requires new solutions to the anti-abuse problem.
5. Limits of Privacy Partitioning
Privacy Partitioning aims to increase user privacy, though as stated
is not a panacea. The privacy properties depend on numerous factors,
including, though not limited to:
* Non-collusion across contexts; and
* The type of information exposed in each context.
We elaborate on each below.
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5.1. Violations by Collusion
Privacy partitions ensure that only the client, i.e., the entity
which is responsible for partitioning, can link all user-specific
information together up to collusion. No other entity individually
knows how to link all the user-specific information as long as they
do not collude with each other across contexts. This is why non-
collusion is a fundamental requirement for privacy partitioning to
offer meaningful privacy for end-users. In particular, the trust
relationships that users have with different parties affects the
resulting impact on the user's privacy.
As an example, consider OHTTP, wherein the Oblivious Relay knows the
Client identity but not the Client data, and the Oblivious Gateway
knows the Client data but not the Client identity. If the Oblivious
Relay and Gateway collude, they can link Client identity and data
together for each request and response transaction by simply
observing requests in transit.
It is not currently possible to guarantee with technical protocol
measures that two entities are not colluding. However, there are
some mitigations that can be applied to reduce the risk of collusion
happening in practice:
* Policy and contractual agreements between entities involved in
partitioning, to disallow logging or sharing of data, or to
require auditing.
* Protocol requirements to make collusion or data sharing more
difficult.
* Adding more partitions and contexts, to make it increasingly
difficult to collude with enough parties to recover identities.
5.2. Violations by Insufficient Partitioning
It is possible to define contexts that contain more than one type of
user-specific information, despite effort to do otherwise. As an
example, consider OHTTP used for the purposes of hiding client-
identifying information for a browser telemetry system. It is
entirely possible for reports in such a telemetry system to contain
both client-specific telemetry data, such as information about their
specific browser instance, as well as client-identifying inforamtion,
such as the client's location or IP address. Even though OHTTP
separates the client IP address from the server via a relay, the
server still learns this directly from the client.
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Other relevant examples of insufficient partitioning include TLS and
Encrypted Client Hello (ECH) [I-D.ietf-tls-esni] and VPNs. TLS and
ECH use cryptographic protection (encryption) to hide information
from unauthorized parties, but both clients and servers (two
entities) can link user-specific data to user-specific identity (IP
address). Similarly, while VPNs hide identity from end servers, the
VPN server has still can see the identity of both the client and
server. Applying privacy partitioning would advocate for at least
two additional entities to avoid revealing both (identity (who) and
user actions (what)) from each involved party.
While straightforward violations of user privacy like this may seem
straightforward to mitigate, it remains an open problem to determine
whether a certain set of information reveals "too much" about a
specific user. There is ample evidence of data being assumed
"private" or "anonymous" but, in hindsight, winds up revealing too
much information such that it allows one to link back to individual
clients; see [DataSetReconstruction] and [CensusReconstruction] for
more examples of this in the real world.
Beyond information that is intentionally revealed by applying privacy
partitioning, it is also possible for information to be
unintentionally revealed through side channels. For example, in the
two-hop proxy arrangement described in Section 3.1, Proxy A sees and
proxies TLS data between the client and Proxy B. While it does not
directly learn information that Proxy B sees, it does learn
information through metadata, such as the timing and size of
encrypted data being proxied. Traffic analysis could be exploited to
learn more information from such metadata, including, in some cases,
application data that Proxy A was never meant to see. Although
privacy partitioning does not obviate such attacks, it does increase
the cost necessary to carry them out in practice. See Section 7 for
more discussion on this topic.
6. Partitioning Impacts
Applying privacy partitioning to communication protocols lead to a
substantial change in communication patterns. For example, instead
of sending traffic directly to a service, essentially all user
traffic is routed through a set of intermediaries, possibly adding
more end-to-end round trips in the process (depending on the system
and protocol). This has a number of practical implications,
described below.
1. Service operational or management challenges. Information that
is traditionally passively observed in the network or metadata
that has been unintentionally revealed to the service provider
cannot be used anymore for e.g., existing security procedures
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such as application rate limiting or DDoS mitigation. However,
network management techniques deployed at present often rely on
information that is exposed by most traffic but without any
guarantees that the information is accurate.
Privacy partitioning provides an opportunity for improvements in
these management techniques with opportunities to actively
exchange information with each entity in a privacy-preserving way
and requesting exactly the information needed for a specific task
or function rather then relying on assumption that are derived on
a limited set of unintentionally revealed information which
cannot be guaranteed to be present and may disappear any time in
future.
2. Varying performance effects and costs. Depending on how context
separation is done, privacy partitioning may affect application
performance. As an example, Privacy Pass introduces an entire
end-to-end round trip to issue a token before it can be redeemed,
thereby decreasing performance. In contrast, while systems like
CONNECT proxying may seem like they would regress performance,
often times the highly optimized nature of proxy-to-proxy paths
leads to improved perforamnce.
Performance may also push back against the desire to apply
privacy partitioning. For example, HTTPS connection reuse
[HTTP2], Section 9.1.1 allows clients to use an existing HTTPS
session created for one origin to interact with different origins
(provided the original origin is authoritative for these
alternative origins). Reusing connections saves the cost of
connection establishment, but means that the server can now link
the client's activity with these two or more origins together.
Applying privacy partitioning would prevent this, while typically
at the cost of less performance.
In general, while performance and privacy tradeoffs are often
cast as a zero sum game, in practice this is often not the case.
The relationship between privacy and performance varies depending
on a number of related factors, such as application
characteristics, network path properties, and so on.
3. Increased attack surface. Even in the event that information is
adequately partitioning across non-colluding parties, the
resulting effects on the end-user may not always be positive.
For example, using OHTTP as a basis for illustration, consider a
hypothetical scenario where the Oblivious Gateway has an
implementation flaw that causes all of its decrypt requests to be
inappropriately logged to a public or otherwise compromised
location. Moreover, assume that the Target Resource for which
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these requests are destined does not have such an implementation
flaw. Applications which use OHTTP with this flawed Oblivious
Gateway to interact with the Target Resource risk their user
request information being made public, albeit in a way that is
decoupled from user identifying information, whereas applications
that do not use OHTTP to interact with the Target Resource do not
risk this type of disclosure.
4. Centralization. Depending on the protocol and system, as well as
the desired privacy properties, the use of partitioning may
inherently force centralization to a select set of trusted
participants. As an example, the impact of OHTTP on end user
privacy generally increases proportionally to the number of users
that exist behind a given Oblivious Relay. That is, the
probability of an Oblivious Gateway determining the client
associated with a request forwarded through an Oblivious Relay
decreases as the number of possible clients behind the Oblivious
Relay increases. This tradeoff encourages centralization of the
Oblivious Relays.
7. Security Considerations
Section 5 discusses some of the limitations of privacy partitioning
in practice. In general, privacy is best viewed as a spectrum and
not a binary state (private or not). Applied correctly, partitioning
helps improve an end-users privacy posture, thereby making violations
harder to do via technical, social, or policy means. For example,
side channels such as traffic analysis
[I-D.irtf-pearg-website-fingerprinting] or timing analysis are still
possible and can allow an unauthorized entity to learn information
about a context they are not a participant of. Proposed mitigations
for these types of attacks, e.g., padding application traffic or
generating fake traffic, can be very expensive and are therefore not
typically applied in practice. Nevertheless, privacy partitioning
moves the threat vector from one that has direct access to user-
specific information to one which requires more effort, e.g.,
computational resources, to violate end-user privacy.
8. IANA Considerations
This document has no IANA actions.
9. Informative References
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[CensusReconstruction]
"The Census Bureau's Simulated Reconstruction-Abetted Re-
identification Attack on the 2010 Census", n.d.,
<https://www.census.gov/data/academy/webinars/2021/
disclosure-avoidance-series/simulated-reconstruction-
abetted-re-identification-attack-on-the-2010-census.html>.
[CONNECT-IP]
Pauly, T., Schinazi, D., Chernyakhovsky, A., Kühlewind,
M., and M. Westerlund, "Proxying IP in HTTP", Work in
Progress, Internet-Draft, draft-ietf-masque-connect-ip-08,
1 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-masque-connect-ip-08>.
[CONNECT-UDP]
Schinazi, D. and L. Pardue, "HTTP Datagrams and the
Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August
2022, <https://www.rfc-editor.org/rfc/rfc9297>.
[DataSetReconstruction]
Narayanan, A. and V. Shmatikov, "Robust De-anonymization
of Large Sparse Datasets", 2008 IEEE Symposium on Security
and Privacy (sp 2008), DOI 10.1109/sp.2008.33, May 2008,
<https://doi.org/10.1109/sp.2008.33>.
[DECOUPLING]
Schmitt, P., Iyengar, J., Wood, C., and B. Raghavan, "The
decoupling principle: a practical privacy framework",
Proceedings of the 21st ACM Workshop on Hot Topics
in Networks, DOI 10.1145/3563766.3564112, November 2022,
<https://doi.org/10.1145/3563766.3564112>.
[DOH] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
[HPKE] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.
[HTTP2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/rfc/rfc9113>.
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[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-15, 3 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-15>.
[I-D.irtf-pearg-website-fingerprinting]
Goldberg, I., Wang, T., and C. A. Wood, "Network-Based
Website Fingerprinting", Work in Progress, Internet-Draft,
draft-irtf-pearg-website-fingerprinting-01, 8 September
2020, <https://datatracker.ietf.org/doc/html/draft-irtf-
pearg-website-fingerprinting-01>.
[ODOH] Kinnear, E., McManus, P., Pauly, T., Verma, T., and C.A.
Wood, "Oblivious DNS over HTTPS", RFC 9230,
DOI 10.17487/RFC9230, June 2022,
<https://www.rfc-editor.org/rfc/rfc9230>.
[OHTTP] Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
Progress, Internet-Draft, draft-ietf-ohai-ohttp-07, 9
March 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-ohai-ohttp-07>.
[PRIVACYPASS]
Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
Pass Architecture", Work in Progress, Internet-Draft,
draft-ietf-privacypass-architecture-11, 6 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-architecture-11>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/rfc/rfc6973>.
Acknowledgments
TODO acknowledge.
Authors' Addresses
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Mirja Kühlewind
Ericsson Research
Email: mirja.kuehlewind@ericsson.com
Tommy Pauly
Apple
Email: tpauly@apple.com
Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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